An EGR system, as is known, recirculates part of the exhaust gas back to the intake of an engine for reducing harmful nitrous oxide emissions. Fuel consumption and engine performance are affected by the recirculated exhaust gas flow. For example, engine performance may be affected by the temperature of the exhaust gas which is higher than that of the fresh air-fuel mixture introduced into the combustion chamber. The "hot" exhaust gas acts to heat up the combustible mixture thus facilitating the combustibility of the air-fuel mixture. As is known, the amount of exhaust gas returned is controlled by an EGR valve that is opened and closed by a control unit depending on operating conditions of the engine. To minimize exhaust gas emissions, it is important to accurately control the amount of exhaust gas recirculated according to engine operating conditions, such as, engine speed, temperature, inlet and exhaust gas pressure and temperature and atmospheric temperature, pressure and moisture conditions. Typically, with a cold start of the engine the EGR valve is initially closed to prevent recirculation, opened immediately after starting to recirculate exhaust gas to more quickly heat the engine and promote more complete combustion of fuel, and then closed when the engine warms up to operating temperature.
Some prior EGR systems have used a sharp edged orifice to limit the maximum flow rate of the recirculated exhaust gas and to provide a single pressure tap upstream of the orifice for a signal used by an engine control processor to indicate when the EGR valve is open. In practice, sharp edge orifices with the same nominal dimensions could not be mass produced with the same flow rate for the same pressure drop and produced significant variations in flow rate and pressure drop from one orifice to another.